1. Field of the Invention
The present invention relates to an image display apparatus, such as a video projector, a TV receiver, and a liquid crystal panel, and more particularly, to a compact image display apparatus suitable for portable use.
2. Background Art
There is a laser realized by combining laser light sources for red, green, and blue that are reduced in size and achieve high luminous efficiencies, and a spatial light modulator such as a liquid crystal element, a scan mirror and a micro mirror, and an image display apparatus using this laser as a light source has been actively developed in recent years. Because the laser is monochromatic, the image display apparatus described above is able to have a color reproduction range 1.2 to 1.7 times wider than that of conventional liquid crystal display projector using color filters, CRT using a fluorescent material, and plasma display, etc. Further, because laser light is readily focused and has linearity, it is possible to reduce an optical system in size. It is therefore expected that a high-definition display, a micro-projector, and the like that have been difficult to achieve will be realized.
As is disclosed in U.S. Pat. No. 3,818,129, the study of a laser display has been conducted from long ago. At the early stage of the study, a gas laser was used as the laser light source in most cases and a scan mirror and a galvanometer mirror were used as a spatial light modulator. It is until recent years that semiconductor lasers for red and blue achieve a high output and that a laser for green using a solid-state laser and a wavelength conversion by the non-linear optics effect and various spatial light modulators have been developed. U.S. Pat. No. 5,255,082 discloses such an image display apparatus formed by combining a laser light source, a scan mirror, and a 1D micro mirror. In addition, U.S. Pat. No. 5,506,597 discloses an image display apparatus using a 2D micro mirror. In U.S. Pat. No. 6,183,092 and U.S. Pat. No. 6,910,774, an image display apparatus using a compact liquid crystal panel and an LCOS (Liquid Crystal On Silicon) are developed and reported.
Regarding the apparatus configuration of the image display apparatus described above, U.S. Pat. No. 6,183,092 and U.S. Pat. No. 6,910,774 disclose a method by which 2D modulation is performed color by color and then beams in the respective colors are combined in a prism. FIG. 24 shows the configuration of a 2D image display apparatus described in the two US patents cited above. Laser light 102 emitted from a laser light source 101 is collimated by a lens 103 and a cylindrical lens 104. The collimated light is scanned on a plane mirror 105 provided in a mirror driving device 106. A scanned beam 107 goes incident on an LCOS element 112 via a field lens 108 and a polarizing prism 109. Of an incident beam 110, a beam 111 whose polarization direction has been rotated passes through the prism plane of the polarizing prism 109 and exits as exiting light 118 by passing through a relay lens 113, an optical path conversion mirror 115, and a projection lens 117.
FIG. 25 schematically shows the configuration of a conventional laser display. Rays of light from respective laser light sources 5100a through 5100c for red, green, and blue are expanded in beam diameter by beam expanders 5102 and go incident on optical integrators 5103. The optical integrators 5103 are homogeneous illumination optical systems that illuminate rectangular openings on spatial light modulators 5107 at uniform illumination intensity. The optical integrators 5103 are of a structure in which two flyeye lenses composed of unit lenses of a rectangular shape arrayed in a 2D lattice are disposed in series.
Rays of light having passed through the optical integrators 5103 illuminate the spatial light modulators 5107 via diffusing plates 5106. Rays of light in respective colors modulated by the spatial light modulators 5107 are combined in a dichroic prism 5109 and formed as an image in full color on a screen 5111 by a projection lens 5110.
Herein, the diffusing plates 5106 reduce speckle noises unique to a laser display apparatus. The laser light sources 5100a through 5100c have a narrow spectrum width and are highly coherent. Hence, rays of light projected and scattered on the screen 5111 interfere with one another randomly, which gives rises to speckle noises like fine particles. The diffusing plates 5106 are formed of a transparent substrate made of polished glass that provides a random phase distribution to the wave front of incident light. By oscillating the diffusing plates 5106 using diffusing plate oscillation means 5113, the phase distribution of light projected onto the screen 5111 varies with the movement of the diffusing plates 5106. As a result, the microscopic pattern of the speckle noises varies as well with time. By oscillating the diffusing plates 5106 so that the pattern of the speckle noises changes faster than an after image time of the viewer, the speckle noises are time-averaged in the eyes of the viewer, which enables a noiseless high-quality image to be perceived.
In the image display apparatus described above, the spatial light modulators 5107 are provided in a one-to-one correspondence with the respective laser light sources 5100a, 5100b, and 5100c for red, green, and blue. Hence, because the homogeneous illumination optical system is necessary for each of the laser light sources 5100a, 5100b, and 5100c, the number of components is large and a component at a high cost, like the dichroic prism, is used. In addition, because the homogeneous illumination optical system occupies a large volume and the diffusing plate oscillation means 5113 is separately necessary for reducing the speckle noises, there arises a problem that the overall apparatus becomes larger.
Meanwhile, different from the method for combining beams in the prism as described above, the field sequential method by which respective light sources are lit ON sequentially to display a color image using an after image in human eyes is now being discussed for the 2D image display apparatus using a light emitting diode as a light source. The former method by which beams are combined in the prism excels in terms of the beauty and brightness of an image. However, the latter field sequential method excels in terms of space saving and the number of components (cost). A pocket projector of a cigarette case size using the field sequential method and formed by combining two scan mirrors and laser light sources for red, green, and blue has been actively developed.
In the field sequential method described above, it is possible to use a micro mirror array represented by a DMD (Digital Micromirror Device) and a liquid crystal on silicon (LCOS) using ferroelectric liquid crystals as the spatial light modulator. These spatial light modulators modulate light by digitally switching ON and OFF light. In a case where the halftone is expressed, the halftone is expressed by changing an ON time of the spatial light modulator with respect to a lighting time of the light source pixel by pixel.
A method for expressing grayscale in a case where the LCOS is used as the spatial light modulator will be described using FIG. 26. The ON/OFF switching of LCOS driving signals a through f is controlled with respect to a lighting time tLD of the light source. In other words, by keeping the driving signal switched ON for 100% of the period of tLD (in the case of the driving signal a), a bright state is achieved. Also, in a case where the ON time is reduced to 0 with respect to tLD by changing the phase of the driving signal (in the case of the driving signal f), a dark state is achieved. The halftone grayscale is achieved by gradually shifting the phase of the ON timing of the driving signal of the LCOS (in the case of the driving signals b through e). A video is formed by performing the operations as above pixel by pixel. These operations are performed not only with the LCOS, but also with the DMD (Digital Micromirror Device) that performs digital modulation.
However, in the 2D image display apparatus using the laser as described above, in a case where the laser light source is lit ON by the field sequential method using the spatial light modulator described above and the scan mirror, problems, such as an increase of a display error in a pixel, contrast deterioration, and noises like a sandstorm, are known to occur in pixels expressing the halftone. These problems possibly become factors that interfere with the realization of the field sequential method that is essential in realizing a compact image display apparatus with excellent portability.
As has been described, attention has been focused on a display apparatus using a laser light source in recent years. In a display using the laser light source, because each ray of light from the laser light source is monochromatic light, it is possible to display an extremely clear image at a high color purity by using a laser light source of an appropriate wavelength. In addition, because the laser light source has high directivity and a focus is achieved efficiently, it is easy to reduce the optical system in size. Further, because it has high photoelectric conversion efficiency, power can be saved in comparison with a conventional lamp light source. From these characteristics, it is possible to realize a more compact display apparatus by using the laser light source, and a portable projector apparatus, such as a pocket projector, is now receiving attention.
However, the field sequential method requires a spatial light modulator with a high display rate. JP-A-5-150209 discloses the configuration of a laser projector by the field sequential method using a single spatial light modulator. According to this configuration, a fixed laser beam is expanded and irradiated to the spatial light modulator. Hence, no consideration is given to a reduction of speckle noises, which makes it impossible to achieve a high-quality image.
There is a ferroelectric liquid crystal element as a spatial light modulator with a high display rate. Different from the conventional element using the nematic liquid crystal phase, the ferroelectric liquid crystal element uses the chiral smectic liquid crystal C phase having spontaneous polarization. In the chiral smectic liquid crystal C phase, the liquid crystal molecules form a layer structure and have spontaneous polarization (PS) in a direction perpendicular to the layer. When an electric field is applied in this direction, the molecules are re-oriented with their spontaneous polarizations being aligned in the direction of the electric field and stay in a bistable state. When combined with a pair of polarization plates (a polarizer and an analyzer), a monochromatic display is achieved. Because the conventional nematic liquid crystal is paraelectric, a response rate induced by application of an electric field is of the order of msec. On the other hand, because the ferroelectric liquid crystal switches by a direct interplay of the spontaneous polarization and the electric field, it is possible to achieve a response rate of the order of μsec, which is an increase of three orders of magnitude. Hence, the ferroelectric liquid crystal element described above is suitable for the field sequential method and performs a grayscale display digitally by the modulation of a time width of a monochrome display. As a product using such a ferroelectric liquid crystal element, there is an LCOS micro display (for example, LV311 available from Displaytech Ltd.) using a semiconductor silicon wafer as a back plane.
Herein, in order to reduce the display apparatus further in size, the homogeneous illumination optical system may be reduced in size. When a laser is used as the light source, because a beam can be made homogenous efficiently by scanning a focused laser beam, it is possible to reduce the homogenous illumination optical system in size. Further, by allowing the beam to pass through the diffusing plate, it is possible to reduce speckle noises at the same time. However, because the ferroelectric liquid crystal element described above displays the grayscale by the time width modulation, there may be a case where it fails to display the grayscale appropriately depending on the timing of a beam scan and a pixel display. It is therefore necessary to use a spatial light modulator for an analog grayscale display. However, a spatial light modulator capable of performing the analog grayscale display has so slow a display rate that it is not suitable for the field sequential method. In short, it is difficult to realize the field sequential method. Hence, there remains a need for the conventional configuration shown in FIG. 25 in which a spatial light modulator is necessary for each laser light source, which makes a reduction of an apparatus in size infeasible.